As is known in the art, low noise oscillators have a wide range of applications such as in navigation, radars and communication systems. As is also known in the art, with transistor oscillators, 1/f noise from the transistors may significantly degrade oscillator phase noise. One technique used to produce low noise oscillators is to screen oscillator transistors for devices having low phase noise. This is time consuming, costly and can sometimes lead to unpredictable yields. Obtaining RF transistors with 1/f noise much less than 1 kHz is desired, but is generally considered impractical. More particularly, RF oscillator phase noise is a dominant factor limiting the performance of many systems. A time based related attribute is the short-term stability or Allan variance. The basic mechanisms of phase noise generation in oscillators are well understood and described in the literature. An example is the model described D. B. Leeson in an article by D. B. Leeson, entitled “A simple model of feedback oscillator noise spectrum,” Proc. IEEE, vol. 54, pp. 329-330, February 1966. This oscillator model is commonly referred to as “Leeson's model”. Many techniques are employed to reduce the phase noise of oscillators, but generally these techniques relate to using transistors with lower 1/f phase noise or higher Q resonators in the feedback circuit.
Phase noise is often described by its spectral properties. For example, phase noise can have a 1/fn characteristic, with n being an integer. For oscillator circuits, n generally varies from 0 to 3. As described in by D. B. Leeson, in the paper entitled “A simple model of feedback oscillator noise spectrum,” Proc. IEEE, vol. 54, pp. 329-330, February 1966, electronic noise within the resonator bandwidth is increased such that 1/f phase noise is converted into 1/f3 phase noise when the device is embedded into a high Q oscillator circuit. The implication of this conversion is that phase noise within the resonator bandwidth is greatly increased. Obtaining lower phase noise then requires either lower 1/f phase noise transistors or higher Q resonators. In particular, the 1/f phase noise of a RE transistor relates to the phase noise at small offset frequencies from the center resonance frequency of the oscillation signal. For example, when referring to 1/f phase noise in a 1 GHz oscillator, the 1/f term applies to noise having a 1/f spectral shape when offset from the 1 GHz output. Although transistor 1/f phase noise is generally attributed to material and surface defects, the precise mechanisms are not well understood.
The origin of 1/f phase noise can be associated with the actual 1/f voltage noise of the transistor, but the specific mechanisms of conversion are also not well understood. One mechanism that converts 1/f voltage noise to 1/f phase noise in bipolar transistors is modulation of collector-to-base capacitance resulting from 1/f voltage noise between the collector and base control electrodes. A similar mechanism that converts 1/f voltage noise to 1/f phase noise in field effect transistors is modulation of source-to-base capacitance resulting from 1/f noise between the source and gate control electrodes. Obtaining RF transistors having very low 1/f phase noise is quite difficult due to compromises between RF performance and 1/f noise.
An analysis was presented by Eva S. Ferre-Pikal, Fred L. Walls, in a paper entitled Guidelines for Designing BIT Amplifiers with Low 1/f AM and PM noise, IEEE Transactions on Ultrasonics, Ferroelectronics and Frequency Control, Vol. 44, No. 2, March 1997 which relates amplifier 1/f phase noise with low frequency voltage fluctuations. Modulation of the collector-base capacitance was proposed as a means of converting 1/f voltage noise to 1/f phase noise.
In a paper entitled Reduction of Phase Noise in Linear HBT Amplifiers Using Low-Frequency Active Feedback by Eva S. Ferre-Pikal, IEEE Transactions on Circuits and Systems, Vol. 51, No. 8, August 2004, the author attempted to stabilize a conventionally biased RF transistor by use of an instrumentation amplifier. The instrumentation amplifier was configured in a conventional topology. There was evidence that additional stability of the transistor bias point could suppress 1/f phase noise. However this topology also introduced several additional resistive components as potential sources of noise and had limited noise suppression. These devices were not embedded into or related to low phase noise oscillators.
The desire is to provide an RF oscillator with very low phase noise, including 1/f phase noise. In addition, it is desired to minimize RF power variations with temperature and process variations.